As used herein the term "ophthalmic lens" means vision correction lenses such as contact lenses and intraocular lenses. Other, less common, vision correction lenses such as artificial corneas and intralamellar implants are also included in this definition.
Bifocal spectacle lenses have been known for hundreds of years. In such lenses a first region of the lens is typically provided with a first focal length while a second region of the lens is provided with a second focal length. The user looks through the appropriate portion of the lens for viewing near or far objects.
More recently there has been interest in developing other types of multifocal ophthalmic lenses. Multifocal contact lenses utilizing an approach similar to that used in spectacle lenses are described in Contact Lenses: A Textbook for Practitioner and Student, Second Edition, Volume 2 on pages 571 through 591. Such lenses have serious drawbacks, however, because they require that the lens shift position on the eye so that different portions of the lens cover the pupil for distant and close vision. This design cannot be used for intraocular lenses or other implanted lenses, because such lenses cannot shift position. Even for contact lenses the design is disadvantageous because it is difficult to insure that the lens will shift properly on the eye for the desired range of vision.
In another design for a bifocal contact lens described in the above-referenced textbook, a central zone of the lens is provided with a first focal length and the region surrounding the central zone is provided with a second focal length. This design eliminates the necessity for shifting the lens by utilizing the phenomenon known as simultaneous vision. Simultaneous vision makes use of the fact that the light passing through the central zone will form an image of a given object at a first distance from the lens and light passing through the outer zone will form an image of the same object at a second distance from the lens. Only one of these image locations will fall on the retina and produce a properly focused image while the other image location will be either in front of or behind the retina. The improperly focused image is so defocused that it will only have the effect of reducing the contrast of the focused image. Since the sensitory response of the eye is logarithmic, a 50 to 60 percent reduction of contrast is barely perceptible and the user of such a lens receives the subjective impression of a single well-focused image.
A disadvantage of such a lens is that, if the central zone is made large enough to provide sufficient illumination in its associated image in low light situations, i.e. when the patient's pupil is dilated, the central zone will occupy all or most of the pupil area when the pupil contracts in a bright light situation. Thus bifocal operation is lost in bright light. Conversely if the central zone is made small enough to provide bifocal operation in bright light situations, an inadequate amount of the light will be directed to the image associated with the central zone in low light environments. Because the central zone is commonly used to provide distant vision, this can create a dangerous situation when the user of such a lens requires distant vision in low light situations such as when the user must drive a motor vehicle at night.
U.S. Pat. Nos. 4,637,697 and 4,642,112 (the Freeman patents) teach a different type bifocal lens in which light is directed to two different focal points by means of refraction and diffraction. A basic refractive power is supplemented by diffractive structures that split the light into a variety of diffractive orders.
A diffractive zone plate must be designed for light of a particular wavelength, .lambda., and will work most efficiently for light of that wavelength. According to conventional design, the radius of the n.sup.th zone (r.sub.n) in the diffractive zone plates must be equal to .sqroot.nr.sub.1 where r.sub.1 is the radius of the central zone. To a reasonable approximation r.sub.1 would be equal to .sqroot.2.lambda.f where .lambda. is the design wavelength and f is the focal length of the diffractive structure. Therefore the n.sup.th zone would have a radius equal to .sqroot.2.lambda.f.
In designing a diffractive zone plate a design wavelength, .lambda., must be selected. When a desired focal length and wavelength are selected, the location of the boundary of each zone, is determined. This rigid definition of the zones results in a disadvantage. If the area of the central zone is too large, under bright light situations with the pupil constricted, only a single zone or very few zones will be utilized. Thus the efficiency of the multi-focal operation is greatly reduced.
An alternative multifocal ophthalmic lens having optical power, at least a portion of the optical power being produced by diffraction, is disclosed by Simpson and Futhey in a commonly assigned U.S. patent application Ser. No., 176,701. The alternative lens also has a plurality of diffractive zones including a circular central zone and a plurality of concentric annular zones. Lenses according to this design meet the condition that r.sub.1.sup.2 -r.sub.0.sup.2 is not equal to r.sub.0.sup.2, where r.sub.0 is the radius of the central zone and r.sub.1 is the radius of the first annular zone. More specifically, r.sub.n is equal to .sqroot.r.sub.0.sup.2 +2n.lambda.)f.
The ophthalmic lenses of the copending U.S. patent application Ser. No. 176,701, now abandoned, utilize designs for which the optical path lengths in adjacent annular zones differ by one-half of the design wavelength. These lenses direct most of the available light energy to two focal points, corresponding to the zeroth and first orders of diffraction, respectively. The zeroth order focal point is used for distant vision applications such as driving, and the first order focal point is used for near vision applications such as reading.
A property of ophthalmic zone plate lenses that utilize diffraction arises from the strong wavelength dependency inherent in the phenomenon of diffraction. Light that goes to the first diffractive order is subject to diffractive chromatic aberration and refractive chromatic aberration. Light to the zeroth order focus is subject only to refractive chromatic aberration. Refractive chromatic aberration and diffractive chromatic aberration are of opposite signs, and if equal, cancel each other out. The normal refractive chromatic aberration of the human eye is about 1 diopter. The diffractive chromatic aberration for a first order diffractive lens with 3 or 4 diopters of add power, is about -1 diopter, hence, for such a lens, the total chromatic aberration is essentially zero for the first order focus. Because there is no diffractive chromatic aberration for the zeroth order focus, its refractive chromatic aberration will not be canceled. Thus, such lenses can correct for chromatic aberration at one focal point, but not both.
Since the zero order of diffraction provides distant vision, while the first order of diffraction provides near vision in prior art lenses, correction of chromatic aberration for such lenses is only available for near vision. In some circumstances, for example, a patient who must drive a motor vehicle in low light conditions, chromatic aberration correction for distant vision would be more desirable.